About 70% of Americans who wear corrective lenses have some degree of astigmatism. It occurs when the cornea (clear covering over the pupil and iris), or the lens inside your eye, has mismatched curves.
The cornea of an eye without astigmatism is shaped like a round ball with a smooth, symmetrical curve that is equal in all directions. This shape focuses light rays precisely onto the back of the retina. If your cornea is not smooth and evenly curved, light rays are scattered rather than focused when they reach the retina and vision is blurry.
If the mismatch is small it doesn’t need to be corrected. When the astigmatism is more than one diopter (1.00D) of optical power, most doctors will offer correction. However, some people find that they are bothered by astigmatisms as small as half a diopter.
Lenticular astigmatism is best correct with glasses, but if you have the more common type of astigmatism that is caused by the irregular shape of your corneas, then toric contacts might be right for you.
How Do Toric Contact Lenses Work?
Toric lenses were developed in 1978 to overcome the shortcomings of regular spherical contact lenses. In a toric lens the shape gradually varies to accommodate the different refractive powers of your cornea.
Toric lenses must stay in their correct placement on your eyes to give you the best correction. There are various designs that accomplish that. One method is to make the lenses thicker at the bottom using a technique called prism ballasting. Another technique is to make the bottom edge straight (truncating) to stabilize the lens in its proper place.
Fitting Toric Contact Lenses
Toric lenses are made in both soft and rigid gas permeable. Soft contact toric lenses are more comfortable, but extra care must be taken in fitting them so they will maintain their proper position.
Because soft toric lenses have various correction fields to accommodate the cornea shape, correctly fitting them is essential to achieving your best possible correction.
With rigid gas permeable lenses only one lens power is used because the layer of tears underneath the lens corrects the astigmatism.
Availability of Toric Lenses
Toric contacts are available in daily, bi-weekly, and monthly disposable varieties. There are also colored toric contact lenses.